Communication systems almost always have a goal of efficiently using the electromagnetic spectrum allocated to them. In order to satisfy this goal, communication systems limit the opportunities for interference. Signals with significantly different frequency or timing parameters do not interfere and may easily be distinguished from one another. Likewise, a strong signal may be distinguished from a relatively weak signal having similar frequency and timing parameters. However, when generally equal strength signals having similar parameters are present, interference is possible. To reduce the likelihood of interference, a communication system often employs constraints which prevent the simultaneous presence of two substantially equal strength signals having substantially the same frequency within the system's area of coverage.
Cellular communication systems have been devised to efficiently use a given spectrum. In conventional cellular systems, an area of coverage is divided into cells. Communication signals are intended to be transmitted and received within the confines of a single cell. Thus, transmission power levels are adjusted as low as possible while still insuring reliable reception within the cell. Adjacent cells are typically assigned different sections of the given spectrum so that no interference occurs between communications in adjacent cells. However, cells that are not adjacent to one another may reuse the same spectrum. Transmission power levels are sufficiently low so that no significant interference problem exists between communications taking place in non-adjacent cells.
A characteristic of cellular systems is that the amount of communication traffic which may be carried by a given spectrum increases as cell size decreases because transmission power decreases correspondingly. As transmission power decreases, the amount of reuse possible for a given spectrum in a given area increases. Thus, it is desirable to have cell sizes as small as possible where communication traffic is great.
On the other hand, larger cell sizes are more desirable where communication traffic is small or where areas of coverage are large. Larger cells provide communication services over greater distances. Likewise, the costs of installing, operating, and maintaining the equipment needed to support only a few large cells are less than the costs for installing, operating, and maintaining many small cells. Furthermore, as subscriber units move relative to cells, the quantity of overhead communications required to handoff calls from one cell to another decreases dramatically with increasing cell sizes.